With rising energy costs, especially high electric costs, and electricity use restrictions placed on heat treating companies in many states and countries, the need to develop more energy efficient heat treating furnace hot zones is a key priority. The furnace hot zone is the area within which a work piece is placed to be heat treated. The present invention includes some notable improvements over prior art hot zone arrangements for saving energy and reducing the overall costs of manufacturing, owning and operating a vacuum furnace. A uniquely designed insulation arrangement, heating elements and their connection joints, and lower mass cooling nozzle size and shape, result in improved energy consumption by the vacuum furnace, improved ease of fabrication and maintenance, and a significant reduction in the initial cost to build the furnace compared with current graphite vacuum furnace hot zones.
It is well known in prior art vacuum furnace fabrication that the hot zone contains an inner insulating wall and an outer wall known as the support ring—U.S. Pat. Nos. 9,187,799; 7,514,035; 4,559,631; 4,259,538; 6,021,155; and US2013/0175256A. The outer wall support ring typically is fabricated as a stainless steel or carbon steel ring and is situated and isolated within a water-cooled chamber. The inner insulating wall typically is fabricated with all metal radiation shields or a combination of graphite felt and foil, or rigidized graphite board. In one instance found in U.S. Pat. No. 4,489,920 ('920 patent), there is described a hot zone insulated by ceramic oxide fabricated boards. It is stated in the patent that the ceramic oxide fabricated boards are much lower in cost and the oxide will not interact with materials that evaporate from the work pieces, as does the graphite felt which the ceramic oxide boards replace. In tests using the ceramic oxide boards claimed in the '920 patent, major catastrophic failures occurred after several repeated process cycles in at least three production furnaces. The ceramic oxide boards were more hygroscopic (water absorbing) than the graphite felt predecessors. This resulted in longer furnace pump-down rates, especially during humid weather, causing lost production time. In the '920 patent the ceramic oxide boards were supported by multiple types of abutment supports, suggesting that the strength of the ceramic oxide boards were less than desired. The use of these multiple supports adds mass to the furnace and is a source of conductive heat loss from the hot zone where the work piece is being treated to the cold side of the support ring, resulting in higher energy usage and costs. In practice after a certain amount of usage the ceramic oxide boards began to fracture and deteriorate rapidly due to thermal shock during high pressure quenching. The weakness of the ceramic oxide boards was found to be due to the fact that the ceramic fibers were not interwoven or interlocked, resulting in a loss of strength when they were exposed to rapid heating and cooling. These two significant failures, extreme moisture absorption and brittleness, led to very costly down time, and repair and replacement costs. Heat treating furnace manufacturers returned to the use of graphite felt and foil insulation, and eventually felt/foil and board combinations in the manufacture of graphite insulated vacuum furnaces.
A major drawback to felt/foil and outer rigid insulation board designs is the need to hold the insulation package in place by retainers to prevent damage and breakage of the woven fibers during high pressure gas quenching. These retainers are typically made from graphite or molybdenum rods that are connected to the face of the insulation package and pass through the insulation to connect to the cold side of the support ring. Each connection from the inside of the inner hot zone wall to the outer support ring is a potential source for thermal losses during the heat treating cycle. The retainer pins according to the present invention are used only in those newly designed insulating board segments which do not have any other connection means to the hot zone support ring. This includes cooling nozzles that screw into the outer support ring, and heating element supports that connect the heating elements to the outer support ring. These three forms of connection means all serve as thermal loss conduits from the hot zone to the support ring, which in turn radiates out to the water-cooled outer chamber wall. Any design that reduces the number of insulation retainer pins helps to improve thermal efficiency in the hot zone. For example, a furnace with a 48 inch hot zone diameter and 50 inches in length may require up to 500 retainer pins as support for the felt/foil insulation package. This results in 500 apertures that are a source of thermal losses due to conduction between the hot zone and the outer support ring. The current design, which utilizes the high strength HEFVAC graphite boards according to the present invention, reduces the number of insulation retainer pins from 17 to 4 around the circumference of the furnace outer support ring. The overall number of retainer pins required according to the prior art designs decreases from approximately 500 to approximately 125. The number of heating element supports is also decreased in the current design from an average of 9 at the circumference to 4, or by greater than 50%.
As vacuum furnaces have improved through the use of high quality seals and valves, issues with oxygen exposure have been virtually eliminated, thus making the statements in the '920 patent, regarding the dangers of graphite felt in vacuum furnaces no longer relevant. It has therefore been the practice for the past 30 plus years to continue to use graphite insulation in vacuum furnaces that utilize graphite or molybdenum electrical resistance heating elements. It is customary for these types of heat treating furnaces to use electrical resistance heating elements, as shown and described in U.S. Pat. Nos. 4,559,631; 4,259,538; and 6,021,155. During the life of a vacuum furnace, the heating elements are subjected to many expansions and contractions as a result of hundreds of heating and cooling cycles. As the state of high pressure gas quenching has advanced, the thermal shock experienced by the heating elements has increased with each increase in pressure levels. Such increases in quench pressures are described in U.S. Pat. No. 9,187,799, where gas quench pressures up to 20 Bar in nitrogen are utilized. The advent of higher heat treating temperatures for specialty alloys has also introduced more stress on the heating elements, leading to increases in the number of failures. The increased stress from higher temperatures and more rapid cooling leads to increased occurrences of fracture of the heating elements, requiring improvements in heating element design for ease of replacement in the heat treating facility, as opposed to replacement in the furnace manufacturing facility. The polygon design shown and described in U.S. Pat. No. 6,021,155, uses a plurality of compensator bars to join straight molybdenum heating elements. Each compensator bar requires 4 nuts and bolts made of refractory material, and has a center aperture which allows connection of the heating element through the insulation package to the hot zone chamber outer wall. For each bank of heating elements there is a 2 to 1 element to retainer pin ratio in this prior art design. Each retainer pin adds to the overall level of conductive heat loss, as each pin is directly connected from the heating element to the hot zone outer stainless steel support ring. Reduction of the number of retainer pins and heating element locking fasteners helps to reduce the overall mass and number of penetrations in the hot zone, thereby reducing energy requirements for heating the hot zone to the required furnace operating temperature. This results in increased furnace efficiency and reduced operating costs.
Another improvement of the present invention over the prior art vacuum furnaces designed to further reduce the overall mass of the hot zone, and thereby increase furnace efficiency, is the design of the cooling nozzles. The current design nozzles are of a reduced size and a streamlined shape, and thus a lower mass when compared with the standard nozzles described and shown in U.S. Pat. Nos. 9,187,799 and 7,514,033.